GERSONIDES (1288–1344), French mathematician and philosopher, known also as Levi Ben Gershom and, in rabbinic texts, by the acronym RaLBaG (Rabbi Levi ben Gershom). Born in Bagnols, Gersonides lived most of his life in Orange and Avignon. Little else is known about him other than where he resided in Provence under the protection of the popes. Gersonides says almost nothing about his personal life, but some scholars have speculated that he may have functioned as a community rabbi, as a banker, or both. Given the nature of his writings and where he lived, it is not unreasonable to speculate that in addition to his involvement with the Jewish community, he may have taught astronomy/astrology in the papal university, medical school, or court. Gersonides is generally acknowledged to be the greatest and most independent medieval Jewish philosopher after the death of Moses Maimonides (Mosheh ben Maimon, 1135/8–1204). Of those rabbis who based their religious thought on the philosophy of Aristotle, Gersonides is the most thorough and rigorous; his major work in this area is The Wars of the Lord (1329). Gersonides also dealt with rabbinics, philosophy, mathematics, medicine, and astronomy.
In rabbinics Gersonides wrote commentaries on the Pentateuch, the Former Prophets, Proverbs, Job, Song of Songs, Ruth, Ecclesiastes, Esther, Daniel, Ezra, Nehemiah, and 1 and 2 Chronicles, as well as a commentary on the thirteen hermeneutic rules of Yishmaʿeʿl ben Elishaʿ (a tanna of the first and second centuries) and a commentary on the tractate Berakhot of the Babylonian Talmud. In philosophy he published a treatise on direct syllogisms and supercommentaries on the middle commentaries and résumés of Ibn Rushd (1126–1198). In medicine he is known to have written a remedy for the gout. In mathematics he composed a treatise on algebra and a commentary on parts of Euclid's Elements.
Finally, Gersonides published a major treatise on astronomy (1340), which Moritz Steinschneider identified as Sefer Tekhunah, which consists of 136 chapters. (A summary of this more detailed work is contained in the second part of the fifth book of The Wars of the Lord.) What is of particular interest to historians of science is that the work contains significant modifications of the systems of Ptolemy and al-Bitruji, as well as useful astronomical tables. The work also includes a description of an instrument, which he calls a magalleh ʿamuqqot (detector of depths), which he invented for making precise astronomical observations. The work was praised and extensively quoted by Giovanni Pico della Mirandola in his Disputationes adversus astrologiam divinatricem (1495). In general Gersonides' instrument is considered the most useful tool developed to assist measurements in astronomy prior to the development of the telescope, and historians of science regard Gersonides as one of the most important European astronomers before Galileo.
The Wars of the Lord deals only with those questions that Maimonides either resolved in direct opposition to Aristotelian principles or explained so obscurely that Maimonides' own view cannot be determined. These questions are discussed in six treatises on, successively, the nature of the soul (i.e., psychology), prophecy (i.e., revelation), God's knowledge, divine providence, the nature of the celestial spheres (i.e., cosmology), and the eternity of matter (i.e., cosmogony).
In each treatise, every question is systematically discussed. First, Gersonides lists all of the different positions that had previously been taken on the issue in question. He then presents a critical analysis of each view, and in so doing lists every form of argument for each position and judges the extent to which each argument is and is not valid. Following this, he states his own view, and he then shows how each of the arguments given for other positions, to the extent to which they are valid, supports his own position. Finally, he demonstrates that his position is in agreement with the correct meaning of the Torah.
Gersonides' theory of divine knowledge was one of the most controversial parts of his work. In the subsequent history of philosophy it led some Jewish thinkers to condemn his work (e.g., Shem Tov ibn Shem Tov, c. 1390–1440) and others to follow him (e.g., Barukh Spinoza, 1632–1677). Gersonides argued that all terms correctly predicated of God and man are such that those terms apply primarily to God and derivatively to humans. Hence, the term-knower refers primarily to how God knows, and by reference to divine knowledge the term is applied to human beings. As their creator, God knows all things as they are essentially in and of themselves. In contrast, human beings, with the assistance of the Active Intellect, know these creations through their senses as effects. God knows everything, but he knows it in a single act of knowledge. The content of divine knowledge is expressible in human terms as an infinite conjunction of distinct universal, conditional propositions. Concerning a specific entity or fact, whereas human beings may know it accidentally, as a particular, through sense reports, God knows it essentially, as a unique individual, through his intellect. Gersonides' opponents interpreted this thesis to amount to a denial that God knows particulars, with the consequence that God is limited in knowledge and power.
Possibly the most original part of Gersonides' work was his cosmology. The concluding treatise of The Wars of the Lord consists of a detailed demonstration, based on astronomy and physics, of the existence of the different heavenly intelligences (angels) and the uniqueness of the ultimate intelligence (God). In terms of its philosophical and scientific elements, this treatise constitutes the most sophisticated work of theology in the history of Judaism. In it, Gersonides argues that this unending universe was created in time, not out of the remains of some previously existing universe but out of nothing. However, the "nothing" from which the world was created is not absolutely nothing; instead, it is an eternal, unformed matter, unlike any other matter of which we can conceive. Gersonides' account of this matter may be the most original part of the work. It is significantly different from the theory of prime matter found in any other work of Jewish, Muslim, or Christian philosophy. But to give an adequate account of it involves a technical discussion that goes beyond the confines of this essay. Suffice it to say that Gersonides' theory of prime matter bears some resemblance to the use by Hermann Cohen (1842–1918) of the term origin in his application of the infinitesimal calculus to ontology, and it may have parallels with the kind of high-energy radiation from which the universe originated, according to those astrophysicists who support the Big Bang theory.
A full list of the published writings of Gersonides can be found in Bernhard Blumenkranz's Auteurs juifs en France médiévale (Toulouse, France, 1975), pp. 65–69. An extensive bibliography of secondary sources is given in Menachem M. Kellner's "Gersonides, Providence, and the Rabbinic Tradition," Journal of the American Academy of Religion 42 (1974): 673–685.
The best source for information about Gersonides' life are two essays by Joseph Shatzmiller, one in Hebrew (in Studies in the History of the Jewish People and the Land of Israel 2 [1972]: 111–126) and the other in French (in Gersonide en son temps, edited by Gilbert Dahan [Louvain, Belgium, and Paris, 1991], pp. 33–43). With reference to English translations of primary sources, Abraham Lassen has published an English translation of Gersonides' commentary on the Book of Job under the title The Commentary of Levi ben Gerson (Gersonides) on the Book of Job (New York, 1946). Furthermore, an English translation of the entire Wars of the Lord has been published by Seymour Feldman (Philadelphia, 1984–1999). In addition, there are three English translations of separate treatises, each of which contains valuable commentaries: on treatise 3, see Norbert M. Samuelson's Gersonides' The War of the Lord, Treatise Three: On God's Knowledge (Toronto, 1977); on treatise 4, see J. David Bleich's Providence in the Philosophy of Gersonides (New York, 1973); and on treatise 6, see Jacob J. Staub's The Creation of the World according to Gersonides (Chico, Calif., 1982).
Gersonides' positions on divine knowledge and providence, as well as his cosmogony, are inherently connected with his cosmology. As yet no one has undertaken the difficult task of translating his treatise on astronomy. However, considerable light on his cosmology is given in the many publications of Bernard R. Goldstein, especially The Astronomy of Levi ben Gerson (New York, 1985).
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